What would happen to a contracting cloud fragmentif it were not able to radiate away its thermal energy?

A. It would continue contracting, but its

temperature would not change.

B. Its mass would increase.

C. Its internal pressure would increase.

Solar system formation is a good example of star birth.

Recall theNebula Model

of the Solar System.

Cloud heats up as gravity causes it to contract due toconservation of energy.Contraction can continue ifthermal energy is radiated away.

As gravity forces a cloud to become smaller, it begins tospin faster and faster, due toconservation of angularmomentum.

As gravity forces a cloud to become smaller, it begins tospin faster and faster, due toconservation of angularmomentum.Gas settles into a spinning disk because spinhampers collapse perpendicular to the spin axis.

Rotation of acontractingcloud speedsup for thesame reason askater speedsup as she pullsin her arms.

Collapse of the Solar Nebula

Collisions between particles in the cloud causeit to flatten into a disk.

These are probably due tomagnetic fields in the newstar. We usually only seethese jets in new stars.

Jets are observed coming from the centers of disks aroundprotostars.

Aprotostar

is the name we give a star that is just forming … i.e. ithas not yet reached the main sequence.

The initial disk that forms around the star is called aprotostellar

disk (also sometimes called aprotoplanetary

disk).

Thought Question

What would happen to a protostar that formedwithout any rotation at all?

A. Its jets would go in multiple directions.

B. It would not have planets.

C. It would be very bright in infrared light.

D. It would not be round.

Protostar to Main Sequence

•A protostar contracts and heats until the core temperatureis sufficient for hydrogen fusion.

•Contraction ends when energy released by hydrogen fusionbalances energy radiated from the surface.

•It takes 30 million years for a star like the Sun (less timefor more massive stars).

•Before it reaches the main sequence the cloud\protostar has a largerradius (it is collapsing) and a lower temperature (fusion has not startedyet) than it will have on the main sequence.So it must approach themain sequence from the right hand side of the HR diagram.

Summary of Star Birth

1.Gravity causes gas cloud to shrink and fragment.

2.Cores of shrinking cloud fragments heat up.

3.Collapse only continues if the cloud cools by radiating away heat.

4.If the initial cloud was spinning a protostellar disk is formed.

5.Protostars approach the main sequence from the right hand side of the HR diagram.

8.New star achieves long-lasting state of balance on the Main Sequence (i.e. thethermostat model that we discussed for the Sun where the rate of nuclear fusionproduced sufficient thermal gas pressure to resist gravitational collapse).

How massive are newborn stars?

A cluster of many stars can form out of a single cloud.

Very massive starsarerare.

Low-mass stars arecommon.

Only about 1 in 200stars is an O typestar, whereas 90%of all stars areeither spectral typeK or M.

Upper Limit on a Star’s Mass

•Photons of light exert a slightamount of pressure when theystrike matter.

•Very massive stars are soluminous that the collectivepressure of photons drives theirmatter into space.

•Hence very large stars are notstable and quickly fall apart doto photon pressure.

Upper Limit on a Star’s Mass

•Models of starssuggest thatradiation pressurelimits how massivea star can be withoutblowing itself apart.

•Observations havenot found stars moremassive than about300MSun.

Lower Limit on a Star’s Mass

•Fusion will not begin in a contracting cloud if somesort of force stops contraction before the coretemperature rises above 107